U.S. patent number 11,002,545 [Application Number 16/546,115] was granted by the patent office on 2021-05-11 for sideband heterodyne switching for resonator fiber optic gyroscopes (rfogs).
This patent grant is currently assigned to Honeywell International Inc.. The grantee listed for this patent is Honeywell International Inc.. Invention is credited to Tiequn Qiu, Glen A. Sanders, Marc Smiciklas, Lee K. Strandjord, Jianfeng Wu.
United States Patent |
11,002,545 |
Strandjord , et al. |
May 11, 2021 |
Sideband heterodyne switching for resonator fiber optic gyroscopes
(RFOGs)
Abstract
Systems and methods for performing SHD switching for RFOGS are
provided herein. A system includes a resonator in which light
resonates; at least one laser source that produces first and second
optical beams; heterodyne modulators that modulate the first and
second optical beams at a heterodyne frequency plus a modulation
frequency offset to produce multiple sideband optical beams,
wherein the modulation frequency offset has a different sign for
the first and second optical beams; a frequency switching
controller that alternatingly switches the signs of the modulation
frequency offset applied to the first and second optical beams,
wherein the heterodyne modulation of the first and second optical
beams are on average at the heterodyne frequency; at least one
coupler that couples the sideband optical beams into the resonator;
a feedback control that detects the sideband optical beams
transmitted from the resonator and, in response, adjusts
frequencies of the optical beams.
Inventors: |
Strandjord; Lee K. (Tonka Bay,
MN), Sanders; Glen A. (Scottsdale, AZ), Wu; Jianfeng
(Tucson, AZ), Qiu; Tiequn (Glendale, AZ), Smiciklas;
Marc (Phoenix, AZ) |
Applicant: |
Name |
City |
State |
Country |
Type |
Honeywell International Inc. |
Morris Plains |
NJ |
US |
|
|
Assignee: |
Honeywell International Inc.
(Charlotte, NC)
|
Family
ID: |
71108377 |
Appl.
No.: |
16/546,115 |
Filed: |
August 20, 2019 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20210055108 A1 |
Feb 25, 2021 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01C
19/727 (20130101); G01C 19/721 (20130101); G01C
19/723 (20130101) |
Current International
Class: |
G01C
19/72 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
European Patent Office, "Extended European Search Report from EP
Application No. 20180633.8", from Foreign Counterpart to U.S. Appl.
No. 16/546,115, dated Nov. 17, 2020, pp. 1 through 9, Published:
EP. cited by applicant.
|
Primary Examiner: Lee; Hwa Andrew
Attorney, Agent or Firm: Fogg & Powers LLC
Government Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with Government support under Ordnance
Technology Base Agreement No. 2016-316, Ordnance Agreement No. 1
awarded by NTA INC PO NTA-CR-DOTC2016316-02. The Government has
certain rights in the invention. This material is based upon work
supported by the DOTC.
Claims
What is claimed is:
1. A system comprising: a resonator configured to allow light to
resonate therein; at least one laser source configured to produce a
plurality of optical beams, the plurality of optical beams
comprising a first optical beam and a second optical beam; a
plurality of heterodyne modulators that perform heterodyne
modulation of the first optical beam and the second optical beam at
a heterodyne frequency plus a modulation frequency offset to
produce a plurality of sideband optical beams at a plurality of
sideband frequencies, wherein the modulation frequency offset has a
different sign for the first optical beam and the second optical
beam; a frequency switching controller that performs sideband
heterodyne switching that alternatingly switches the signs of the
modulation frequency offset applied to the first optical beam and
the second optical beam, wherein the heterodyne modulation of the
first optical beam and the second optical beam are on average
substantially at the heterodyne frequency during a period of time;
at least one coupler configured to couple the plurality of sideband
optical beams into the resonator; and a feedback control coupled to
the resonator that detects the plurality of sideband optical beams
transmitted out of the resonator and adjusts frequencies of the
plurality of optical beams based on the detected plurality of
sideband optical beams.
2. The system of claim 1, wherein the first optical beam and the
second optical beam are generated to propagate within the resonator
at different resonant modes.
3. The system of claim 2, wherein the frequency switching
controller periodically performs resonant mode switching, wherein
the resonant mode switching switches resonant modes of the first
optical beam and the second optical beam.
4. The system of claim 3, wherein the frequency switching
controller performs the sideband heterodyne switching and the
resonant mode switching at the same period.
5. The system of claim 3, wherein the frequency switching
controller performs the sideband heterodyne switching and the
resonant mode switching at different periods that are harmonically
related to one another.
6. The system of claim 1, wherein the plurality of heterodyne
modulators are located on a silicon photonics chip.
7. The system of claim 1, wherein the feedback control demodulates
the detected plurality of sidebands at a demodulation frequency
that is twice of the combination of the heterodyne frequency and
the corresponding modulation frequency offset.
8. The system of claim 1, wherein the modulation frequency offset
is applied to the heterodyne frequency by modulating the heterodyne
frequency by an offset frequency.
9. The system of claim 1, wherein frequencies of the first optical
beam and the second optical beam are locked to a reference
frequency of a master laser.
10. A method comprising: generating a plurality of optical beams,
the plurality of optical beams comprising a first optical beam and
a second optical beam; performing heterodyne modulation of the
first optical beam and the second optical beam at a heterodyne
frequency plus a modulation frequency offset to produce a plurality
of sideband optical beams at a plurality of sideband frequencies,
wherein the modulation frequency offset has a different sign for
the heterodyne modulation of the first optical beam than the
modulation of the second optical beam; alternating the signs of the
modulation frequency offset applied to the heterodyne modulation of
the first optical beam and the second optical beam, wherein the
heterodyne modulation modulates the first optical beam and the
second optical beam substantially on average at the heterodyne
frequency during a period of time; circulating the plurality of
sideband optical beams in a resonator; detecting the plurality of
sideband optical beams transmitted out of one or more ports of the
resonator; adjusting the frequencies of the generated plurality of
optical beams based on the detected plurality of sideband optical
beams.
11. The method of claim 10, wherein generating the plurality of
optical beams comprises generating the first optical beam and the
second optical beam to propagate within the resonator at different
resonant modes.
12. The method of claim 11, further comprising periodically
performing resonant mode switching, wherein the resonant mode
switching switches carrier frequencies of the first optical beam
and the second optical beam.
13. The method of claim 12, further comprising performing the
sideband heterodyne switching and the resonant mode switching at
the same period.
14. The method of claim 12, further comprising performing the
sideband heterodyne switching and the resonant mode switching at
different periods that are harmonically related to one another.
15. The method of claim 10, wherein the plurality of heterodyne
modulators are located on a silicon photonics chip.
16. The method of claim 10, further comprising demodulating the
detected plurality of sidebands at a demodulation frequency that is
twice of the combination of the heterodyne frequency and the
corresponding modulation frequency offset.
17. The method of claim 10, performing heterodyne modulation of the
first optical beam and the second optical beam comprises modulating
the heterodyne frequency by a respective offset frequency.
18. The method of claim 10, further comprising locking the
frequencies of the first optical beam and the second optical beam
to a reference frequency of a master laser.
19. A resonator fiber-optic gyroscope comprising: a resonator
configured to allow light to resonate therein, wherein the
resonator has a plurality of resonant modes, each resonant mode
separated by a free spectral range; a first laser source that
produces a first optical beam having a first frequency; a first
heterodyne modulator to modulate the first optical beam at a first
offset heterodyne frequency to produce first sideband signals that
are offset by the first offset from first sideband resonant modes
in the plurality of resonant modes, wherein the first sideband
signals propagate in the resonator in a first direction; a second
laser source that produces a second optical beam having a second
frequency; a second heterodyne modulator to modulate the second
optical beam at a second offset heterodyne frequency to produce
second sideband signals that are offset by the second offset from
second sideband resonant modes in the plurality of resonant modes,
wherein the second sideband signals propagate in the resonator in a
second direction that is opposite to the first direction; a first
feedback control configured to: detect the first sideband signals
received from a first port of the resonator to produce a first
detected signal; demodulate the first detected signal to form a
first demodulated signal; and adjust the first frequency such that
the first sideband signals move closer to being offset by the first
offset from the first sideband resonant modes based on the first
demodulated signal; and a second feedback control configured to:
detect the second sideband signal received from a second port of
the resonator to produce a second detected signal; demodulate the
second detected signal to form a second demodulated signal; and
adjust the second frequency such that the second sideband signals
move closer to being offset by the first offset from the second
sideband resonant modes based on the second demodulated signal; a
frequency switching controller that periodically switches the first
offset to a value of the second offset and the second offset to a
value of the first offset.
20. The resonator fiber-optic gyroscope of claim 19, wherein the
frequency switching controller periodically swaps the first
sideband resonant modes and the second sideband resonant modes.
Description
BACKGROUND
The resonator fiber optic gyroscope (RFOG) may potentially provide
high rotation sensing performance within a small volume at low
cost. The RFOG may use at least two laser beams, where at least one
laser beam propagates around a resonator coil in the clockwise (CW)
direction and at least one other laser beam propagates in the
counterclockwise (CCW) direction. In meeting gyroscope performance
requirements, the center frequency of the CW and CCW resonances may
be measured at a high degree of precision. In some implementations,
the RFOGs may use phase or frequency modulation of the CW and CCW
laser frequencies and demodulation of the resonator output to
precisely detect the resonance center frequency.
In operation, RFOGs may have at least two types of gyroscope bias
errors. These bias errors include modulation imperfections and
optical backscatter. Further, some solutions that resolve the
modulation imperfection errors may exacerbate the optical
backscatter errors and vice versa. For example, modulation
imperfection errors may be suppressed by using common modulation
for both the CW and CCW propagating laser beams. Conversely,
optical backscatter errors may be suppressed by using separated
modulation at different frequencies for the CW and CCW propagating
laser beams. Sideband heterodyne detection (SHD) is a method that
uses common modulation along with SHD modulation at a very
high-frequency to suppress both modulation imperfection errors and
optical backscatter errors.
SUMMARY
Systems and methods for performing sideband heterodyne switching
for resonator fiber optic gyroscopes are provided herein. In
certain embodiments, a system includes a resonator configured to
allow light to resonate therein. Additionally, the system may
include at least one laser source configured to produce a plurality
of optical beams, the plurality of optical beams comprising a first
optical beam and a second optical beam. Further, the system
includes a plurality of heterodyne modulators that perform
heterodyne modulation of the first optical beam and the second
optical beam at a heterodyne frequency plus a modulation frequency
offset to produce a plurality of sideband optical beams at a
plurality of sideband frequencies, wherein the modulation frequency
offset has a different sign for the first optical beam and the
second optical beam. Also, the system includes a frequency
switching controller that performs sideband heterodyne switching
that alternatingly switches the signs of the modulation frequency
offset applied to the first optical beam and the second optical
beam, wherein the heterodyne modulation of the first optical beam
and the second optical beam are on average substantially at the
heterodyne frequency during a period of time. Moreover, the system
includes at least one coupler configured to couple the plurality of
sideband optical beams into the resonator. Further, the system
includes a feedback control coupled to the resonator that detects
the plurality of sideband optical beams transmitted out of the
resonator and adjust frequencies of the plurality of optical beams
based on the detected plurality of sideband optical beams.
DRAWINGS
Understanding that the drawings depict only some embodiments and
are not therefore to be considered limiting in scope, the exemplary
embodiments will be described with additional specificity and
detail using the accompanying drawings, in which:
FIG. 1 is a block diagram illustrating an exemplary resonator fiber
optic gyroscope that employs sideband heterodyne switching
according to an aspect of the present disclosure;
FIG. 2 is a block diagram illustrating an exemplary resonator fiber
optic gyroscope that employs sideband heterodyne and resonance mode
switching according to an aspect of the present disclosure;
FIG. 3A-3D is a diagram illustrating multiple frequency graphs
showing various combinations of sideband heterodyne frequencies and
resonance frequencies according to an aspect of the present
disclosure;
FIG. 4 is a block diagram illustrating an exemplary resonator fiber
optic gyroscope that employs sideband heterodyne according to an
aspect of the present disclosure; and
FIG. 5 is a flowchart diagram illustrating a method for performing
sideband heterodyne switching according to an aspect of the present
disclosure.
In accordance with common practice, the various described features
are not drawn to scale but are drawn to emphasize specific features
relevant to the example embodiments.
DETAILED DESCRIPTION
In the following detailed description, reference is made to the
accompanying drawings that form a part hereof, and in which is
shown by way of illustration specific illustrative embodiments.
However, it is to be understood that other embodiments may be
utilized and that logical, mechanical, and electrical changes may
be made.
Systems and methods for performing sideband heterodyne switching
for RFOGs are provided herein. As stated above sideband heterodyne
detection may be used to suppress both modulation imperfections and
optical backscatter errors. However, in order to suppress
backscatter errors, the CW and CCW SHD modulation frequencies may
be separated. The frequency separation may lead to some residual
errors due to SHD modulation imperfections, which may increase
significantly when low-cost silicon photonics (SiP) chips are used
for the laser source and to provide the SHD modulation.
In certain embodiments, to suppress the residual errors due to SHD
modulation imperfections, the SHD modulation frequencies may be
further modulated such that the CW and CCW SHD frequencies are
mostly different at any one time, but on average, over time, are at
the proper frequency. By switching the SHD modulation frequencies
in this way, the CW and CCW SHD modulation frequencies are never
the same, and therefore, optical backscatter errors are suppressed,
but the SHD modulation frequencies averaged over time are at or
very near to the SHD proper frequencies and therefore, SHD
modulation imperfections may also be suppressed.
FIG. 1 is a block diagram illustrating an exemplary RFOG 101 that
employs sideband heterodyne switching according to at least one
embodiment of the present disclosure. The RFOG 101 may include a
first laser source 121 and a second laser source 135 that are both
coupled to a fiber optic resonator 149 by at least one optical
coupler 125. The RFOG 101 may also include CCW resonance tracking
electronics 111, CW resonance tracking electronics 147, and
frequency switching controller 131, each of which are discussed in
greater detail below.
As shown in FIG. 1, a laser source 121 outputs a first optical beam
115 that is coupled into the resonator 149 by the coupler 125 in a
first direction. For the example of FIG. 1, the first optical beam
115 is defined as traveling around the resonator 149 in a
counterclockwise (CCW) direction. A laser source 135 may output a
second optical beam 117 of laser light that is coupled into the
resonator 149 by the coupler 125 and travels around the resonator
149 in a second direction that is opposite to the first direction
traveled by the optical beam 115. For the example of FIG. 1, the
second optical beam 117 is defined as traveling around the
resonator 149 in a clockwise (CW) direction. While the first
optical beam 115 is described as traveling in the CCW direction and
the second optical beam 117 is described as traveling in the CW
direction, the first optical beam 115 may propagate through the
resonator 149 in any direction that is opposite to the propagation
direction of the second optical beam 117.
In certain embodiments, the first laser source 121 and the second
laser source 135 may each be controlled by respective feedback
control, such as the resonance tracking electronics (shown at 111
and 147) to maintain the frequencies of the first optical beam 115
and the second optical beam 117 at resonance frequencies for the
resonator 149. For example, the first laser source 121 may launch
the optical beam 115 into the resonator 149 at a specific optical
frequency (shown in FIG. 1 as fccw). At that frequency, fccw, the
optical beam 115 may exhibit a specific wavelength, .lamda.ccw, in
the resonator (which for laser light may be a wavelength on the
order of 1.5 microns, for example). When the optical beam 115 is
tuned to a frequency fccw such that the resonator 149 length equals
to exactly an integer multiple of wavelengths .lamda.ccw, then the
first optical beam 115 may be operating at a resonant frequency of
the resonator 149 (which may also be referred to as one of the
resonant modes of the resonator 149). At this resonant frequency,
with each pass that the first optical beam 115 travels around the
loop of the resonator 149, the first optical beam 115 is in phase
with the previous passes of the first optical beam 115 around the
resonator 149. Accordingly, the optical power of the first optical
beam 115 from each pass through the resonator 149 accumulates to a
peak resonant intensity. As the frequency fccw deviates from the
resonant frequency, the optical power within the resonator 149 that
is associated with the first optical beam 115 may sum to less than
the peak resonant intensity.
In a similar manner, the second laser source 135 may launch the
second optical beam 117 into the resonator 149 at a specific
optical frequency (shown in FIG. 1 as fcw). At that frequency, fcw,
the second optical beam 117 may exhibit a specific wavelength,
.lamda.cw. When the second optical beam 117 is tuned to a frequency
fcw such that exactly an integer multiple of wavelengths .lamda.cw
are propagating around the resonator 149, then the second optical
beam 117 may be operating at a resonant frequency of the resonator
149 (which may also be referred to as one of the resonant modes of
the resonator 149). At this resonant frequency, with each pass that
the first optical beam 117 travels around the loop of the resonator
149, the first optical beam 117 may be in phase with the previous
passes of the first optical beam 115 around the resonator 149.
Accordingly, the optical power of the first optical beam 115 from
each pass through the resonator 149 accumulates to a peak resonant
intensity. As the frequency fccw deviates from the resonant
frequency, the optical power within the resonator 149 that is
associated with the first optical beam 115 may sum to less than the
peak resonant intensity.
With embodiments of the present disclosure, the first laser source
121 and the second laser source 135 may be controlled by the
corresponding resonance tracking electronics 111 and 147 to remain
locked to different resonance modes with respect to each other.
That is, if the first optical beam 115 is locked to a first
resonant frequency (where an integer number, I, of wavelengths are
propagating in the CCW direction around the resonator 149), then
the second optical beam 117 may be locked to a second resonant
frequency (where an integer number, J, where J.noteq.I, of
wavelengths are propagating in the CW direction around the
resonator 149). The frequencies of beam 115 and beam 117 may be
separated from each other based on a function of the free spectral
range (FSR) of the resonator 149. As such, when the first resonant
frequency is less than the second resonant frequency by exactly one
free spectral range, then the second optical beam 117 may be said
to be operating at the next higher resonant mode than the first
optical beam 115, and the first optical beam 115 may be said to be
operating at the next lower resonant mode than the second optical
beam 117.
In certain embodiments, the first optical beam 115 may be driven to
a frequency fccw that is substantially equal to the first resonant
frequency that corresponds to a first resonant mode. Additionally,
the second optical beam 117 may be driven to a frequency fcw that
is substantially equal to the second resonant frequency that
corresponds to a second resonant mode. Accordingly, the frequency
difference between the peak resonant intensity at the first
resonant mode and the peak resonant intensity at the second
resonant mode may be equal to the free spectral range. In examples
discussed within this disclosure, the first laser source 121 and
the second laser source 135 may operate at adjacent resonant modes
that are separated by one free spectral range. However, it should
be appreciated that additional embodiments may be conceived that
are within the scope of the present disclosure where the first
laser source 121 and the second laser source 135 produce optical
beams separated by other integer multiples of the free spectral
range.
As mentioned above, the frequency fccw of the first optical beam
121 may be locked to a resonant frequency by the CCW resonance
tracking electronics 111. Similarly, the frequency fcw of the
second optical beam 135 may be locked to a resonant frequency by
the CW resonance tracking electronics 147. In some embodiments, the
locking of the optical beams 121 and 135 by associated resonance
tracking electronics 111 and 147 may be performed by operating the
resonance tracking electronics 111 and 147 as frequency locked
loops. More specifically, the CCW optical beam 115 may be frequency
or phase modulated to interrogate the resonator. A portion of the
CCW propagating optical beam 115 may be coupled out of the
resonator 149 by an optical coupler 127 and delivered to a first
photodetector 119, where the first photodetector 119 measures the
transmitted optical intensity of the first output optical beam 116.
From this measurement, the photodetector 119 may produce a
resonance tracking signal, which is an electrical signal that
varies as a function of the measured optical intensity.
In certain embodiments, when the carrier/center optical frequency
of the first optical beam 115 is on resonance, the output of the
photodetector 119 may not have a frequency component at the
modulation frequency. The photodetector output at the modulation
frequency may be proportional to small average optical frequency
deviations from the resonance frequency. Deviations from the first
resonance frequency may produce a tracking error at the modulation
frequency in the resonance tracking signal produced by the
photodetector 119. The CCW resonance tracking electronics 111 may
receive the resonance tracking signal from the photodetector 119 at
the modulation frequency and may provide a control signal to the
laser source 121 that adjust the frequency fccw of the first
optical beam 115 to drive the tracking error at the modulation
frequency to zero (i.e., the control signal provided by the CCW
resonance tracking electronics 111 may drive the optical beam 115
to the desired resonance frequency).
In similar embodiments, a portion of the CW propagating optical
beam 117 may be coupled out of the resonator 149 by the optical
coupler 127 and delivered to a second photodetector 133, which may
measure the transmitted optical intensity of the second output
optical beam 118. From this measurement, the photodetector 133 may
produce a second resonance tracking signal, which is an electrical
signal that varies as a function of the measured optical intensity.
When the carrier/center optical frequency of the CW beam 117 is on
resonance the output of the photodetector 133 may not have a
frequency component at the modulation frequency. The photodetector
output at the modulation frequency may be proportional to small
average optical frequency deviations from the resonance frequency.
Deviations from the second resonance frequency may produce a
tracking error at the modulation frequency reflected in the second
resonance tracking signal. The CW resonance tracking electronics
147 may receive the second resonance tracking signal and provide a
second control signal to the second laser source 135 and adjust the
frequency fcw of the second optical beam 117 to drive the tracking
error at the modulation frequency (i.e., the control signal
provided by the CW resonance tracking electronics 147 may drive the
optical beam 117 to the desired resonance frequency).
In some implementations, when locking the propagating optical beams
within the RFOG 101 to resonant frequencies, the RFOG 101 may
experience different types of bias errors. For example, the RFOG
101 may experience modulation imperfections and optical
backscatter. These errors may be suppressed using different
techniques. For example, the modulation imperfections may be
suppressed using common modulation and the optical backscatter
errors may be suppressed separated modulation at different
frequencies for the optical beams propagating in different
directions within the resonator 149.
In several embodiments, common modulation may be used to suppress
modulation imperfections. While systems for providing common
modulation are not shown in FIG. 1, common modulation of the
optical frequency or phase of each optical beam may be used to
provide an error signal that indicates when the optical frequency
of a beam has deviated away from a resonance for a resonator 149.
Common modulation may produce a resonator output signal at the
photodetectors 119 and 133. The resonator output signal may have a
component at the modulation frequency that may be used as an error
signal to detect when the corresponding laser beam is on resonance.
The amplitude of the error signal at the modulation frequency will
be zero when the frequency of the optical beam is on resonance. If
the frequency of the optical beam deviates from resonance, the
amplitude of the error signal will be non-zero and will have a sign
that depends on the direction of deviation from resonance. The
error signal at the modulation frequency may be demodulated down to
DC by mixing with a reference signal at the modulation frequency.
The DC error signal may then be used by a servo to control the
frequency of the optical beam.
However, because the resonance detection modulation frequency is
common for all the optical beams, a method may be employed to
distinguish the error signal associated with each field. To
distinguish the resonance detection error signals of each field the
RFOG 101 may include multiple phase modulators, such as a first
phase modulator 113 and a second phase modulator 137. The first
phase modulator 113 and the second phase modulator 137 may modulate
the first optical beam 115 and the second optical beam 117 at
different modulation frequencies such that each optical beam has
corresponding sidebands at different locations in the frequency
spectrum.
In some embodiments, the frequencies of the phase modulations may
be referred to as the sideband heterodyne frequencies the phase
modulators 113 and 137 may be referred to as heterodyne modulators
113 and 137. In some implementations, the heterodyne modulation
frequencies are high and equal to an half-integer (n+1/2) of the
free spectral range. The frequency of the optical beams may be
tuned to either place the carrier and even sidebands on resonance,
or the frequencies of the optical beams may be tuned to place the
odd sidebands on resonance. When the odd sidebands are placed on
resonance, the carrier and even sideband of the optical beams will
be off resonance.
In certain embodiments, the first optical beam 115 produced by the
first laser source 121 and the second optical beam 117 produced by
the second laser source 135 may be respectively modulated by a
first heterodyne modulator 113 and a second heterodyne modulator
137. The first heterodyne modulator 113 and the second heterodyne
modulator 137 modulate the first optical beam 115 and the second
optical beam 117 by sideband heterodyne frequencies. For example,
the first heterodyne modulator 113 may modulate the first optical
beam 115 by a first heterodyne frequency 123 that is represented by
f.sub.SHD-,+. The second heterodyne modulator 137 may modulate the
second optical beam 115 by a second heterodyne frequency 129 that
is represented by f.sub.SHD+,-.
In some embodiments, the first heterodyne frequency 123 and the
second heterodyne frequency 129 may be set to be substantially
equal to odd multiples of half of the free spectral range of the
resonator 149. When the first optical beam 115 and the second
optical beam 117 are modulated at the first heterodyne frequency
123 and the second heterodyne frequency 129, the resonance tracking
electronics 111 and 147 may tune a carrier frequency for the first
optical beam 115 and the second optical beam 117 to a frequency in
the middle of two adjacent resonant modes of the resonator 149.
Accordingly, the modulation of the first optical beam 115 and the
second optical beam 117 may produce sideband signals, where the
first harmonic sidebands are located on resonant peaks of the
resonator 149.
In certain embodiments, when the first optical beam 115 and the
second optical beam 117 are modulated at the first heterodyne
frequency 123 and a second heterodyne frequency 129, the sideband
signals produced by the modulation may propagate around the
resonator 149 in opposite directions. For example, the sidebands of
the first optical beam 115 may be coupled into the resonator 149
through the coupler 125. The sidebands of the first optical beam
115 may then propagate around the resonator 149 in the CCW
direction. In a similar fashion, the sidebands of the second
optical beam 117 may also be coupled into the resonator 149 through
the coupler 125. The sidebands of the second optical beam 117 may
then propagate around the resonator 149 in the CW direction.
In some embodiments, when the sideband signals of the second
optical beam 117, propagating in the CW direction, are received by
the coupler 127, a portion of the sideband signals of the second
optical beam 117 are coupled out of the resonator 149 and received
by the photodetector 133. The photodetector 133 may detect the
optical beat note generated by the interference between the
sideband signals and may pass an electrical signal representing the
beat note between the first sideband signals to the CW resonance
tracking electronics 147.
Similarly, when the sideband signals of the first optical beam 115,
propagating in the counterclockwise direction, are received by the
coupler 127, a portion of the sideband signals of the first optical
beam 115 may be coupled out of the resonator 149 and received by
the photodetector 119. The photodetector 119 may detect the optical
beat note generated by the interference between the sideband
signals and may pass an electrical signal representing the beat
note between the first sideband signals to the CCW resonance
tracking electronics 111.
In certain embodiments, the first photodetector 119 and the second
photodetector 133 respectively provide electrical signals to the
CCW resonance tracking electronics 111 and the CW resonance
tracking electronics 147. As described herein, the CCW resonance
tracking electronics 111 and the CW resonance tracking electronics
147 function substantially similar to one another. As shown, the
CCW resonance tracking electronics 111 include an analog-to-digital
converter (ADC) 103. As described herein, the ADC 103 may receive
the analog electrical signal provided by the first photodetector
119 and convert the analog signal to a digital signal.
In some embodiments, the CCW resonance tracking electronics 111 may
include a demodulator 109 coupled to the ADC 103 to receive and
demodulate the digital signal produced by the ADC 103. The
demodulator 109 demodulates the digital signal at a received
demodulation frequency. In some embodiments, the demodulation
frequency is two times the first heterodyne frequency 123. For
example, since the frequency separation between the first sideband
signals, received at the photodetector 119, are at twice the (SHD)
modulation frequency 123, the beat note between the first sidebands
will be at twice the first heterodyne frequency 123.
In further embodiments, where the first optical beam 115 was
commonly modulated, the signal produced by the demodulator 109 may
be demodulated by an additional common demodulator (not shown in
FIG. 1 but illustrated as common demodulator 451 in FIG. 4), where
the common demodulator demodulates the signal produced by the
demodulator 109 at the common modulation frequency.
In certain embodiments, the demodulated signal may be provided to a
servo 107. As described herein, the servo 107 functions to lock the
frequency of the carrier of the first optical beam 115 produced by
the first laser source 121 to either a resonance frequency or in
the middle of two adjacent resonances of the resonator 149. For
example, the servo 107 may track the error (i.e., deviation from
the resonance frequency) in the signal received by the servo 107
and based on the error in the signal, the servo 107 may provide an
output signal to a frequency generator 105. The output signal may
control adjustments made by the frequency generator 105 to the
frequency of the first optical beam 115 such that the sidebands of
the first optical beam 115 produced by the first laser source 121
are at the resonant frequency for the resonator 149. As used
herein, the frequency generator 105 may provide the signal to the
first laser source 121 that instructs the first laser source 121 as
to the frequency of the first optical beam 115 produced by the
first laser source 121. Accordingly, the CCW resonance tracking
electronics 111 are able to determine departures of the first
optical beam 115 from resonant frequencies of the resonator 149 and
provide a signal to the first laser source 121 to adjust the
frequency of the first optical beam 115. Based on the control
signal received from the servo 107, the frequency generator 105 may
provide a signal to the first laser source 121. Also, the servo 107
may provide a measurement of the deviation from the resonant
frequency to a processor, where the processor may determine
rotation rates based on the deviation from the resonant frequency.
In some implementations, the processor may aid the servo 107 in
calculating deviations from the resonant frequencies for the
resonator 149.
As described above, the CW resonance tracking electronics 147
function similarly to the CCW resonance tracking electronics 111.
Thus, the CW resonance tracking electronics 147 include an
analog-to-digital converter (ADC) 143, where the ADC 143 may
receive the analog electrical signal provided by the second
photodetector 133 and convert the analog signal to a digital
signal.
In some embodiments, the CW resonance tracking electronics 147 may
include a demodulator 139 coupled to the ADC 143 to receive and
demodulate the digital signal produced by the ADC 143. The
demodulator 139 demodulates the digital signal at a received
demodulation frequency. In some embodiments, the demodulation
frequency is two times the second heterodyne frequency 129. For
example, since the frequency separation between the first sideband
signals, received at the second photodetector 133, are at twice the
second heterodyne frequency 129, the beat note between the first
sidebands will be at twice the second heterodyne frequency 129.
In further embodiments, where the second optical beam 117 was
commonly modulated, the signal produced by the demodulator 139 may
be demodulated by an additional common demodulator (not shown in
FIG. 1 but illustrated as common demodulator 423 in FIG. 4), where
the common demodulator demodulates the signal produced by the
demodulator 139 at the common modulation frequency, which is the
same common modulation frequency used to demodulate the signal
produced by the demodulator 109 in the CCW resonance tracking
electronics 111.
In certain embodiments, the demodulated signal may be provided to a
servo 145. As described herein, the servo 145 functions to lock the
frequency of the second optical beam 117 produced by the second
laser source 135 to a resonance frequency of the resonator 149. For
example, the servo 145 may track the error (i.e., deviation from
the resonance frequency) in the signal received by the servo 145
and based on the error in the signal, the servo 145 may provide an
output signal to a frequency generator 141. The output signal may
control adjustments made by the frequency generator 141 to the
frequency of the second optical beam 117 such that the sidebands of
the second optical beam 117 produced by the second laser source 135
are at the resonant frequency for the resonator 149. As used
herein, the frequency generator 141 may provide the signal to the
second laser source 135 that instructs the second laser source 135
as to the frequency of the second optical beam 117 produced by the
second laser source 135. Accordingly, the CW resonance tracking
electronics 147 can determine departures of the second optical beam
117 from resonant frequencies of the resonator 149 and provide a
signal to the second laser source 135 to adjust the frequency of
the second optical beam 117. Based on the control signal received
from the servo 145, the frequency generator 141 may provide a
signal to the second laser source 135. Also, the servo 145 may
provide a measurement of the deviation from the resonant
frequencies to a processor, where the processor may determine
rotation rates based on the deviation from the resonant
frequencies. In some implementations, the processor may aid the
servo 145 in calculating deviations from the resonant frequencies
for the resonator 149.
As described above, modulation imperfection errors may be
suppressed using common modulation for both the first optical beam
115 and the second optical beam 117. Additionally, optical
backscatter errors may be suppressed by using separated modulation
at different frequencies (sideband heterodyne frequencies) for the
first optical beam 115 and the second optical beam 117. To suppress
the backscatter errors, the first sideband heterodyne frequency 123
and the second sideband heterodyne frequency 129 may be separated
in frequency. However, this frequency separation may lead to some
residual errors due to sideband heterodyne modulation
imperfections. In particular, the residual errors increase
significantly when low-cost silicon photonics (SiP) chips are used
for the first laser source 121, the second laser source 135, and
the performance of the sideband heterodyne modulation. Accordingly,
systems described herein may further suppress errors from sideband
heterodyne modulation imperfections, which may facilitate the use
of low cost SiP modulators for performing the sideband heterodyne
modulation.
The amplitude of rotation sensing errors from sideband heterodyne
modulation imperfections may be proportional to the sideband
heterodyne modulation frequency deviation from the proper sideband
heterodyne frequency. As described above, the proper sideband
heterodyne frequency may occur when the sideband heterodyne
modulation frequency is at an odd integer of half the free spectral
range for the resonator 149. Thus, sideband heterodyne modulation
imperfection errors go to zero when the sideband heterodyne
modulation frequency is at the proper frequency. However, if both
the CW and CCW sideband heterodyne frequencies are set at the
proper frequency, then optical backscatter errors may become very
large.
In certain embodiments, to reduce optical backscatter errors that
arise from the CW and CCW sideband heterodyne frequencies being
both set to the proper sideband heterodyne modulation frequency,
the first sideband heterodyne frequency 123 and the second sideband
heterodyne frequency 129 are shifted by an offset frequency such
that the first sideband heterodyne frequency 123 and the second
sideband heterodyne frequency 129 are mostly different at any one
time, but on average over time are at the proper heterodyne
frequency.
In some embodiments, the first sideband heterodyne (SHD) frequency
123 (f.sub.SHD-,+) and the second SHD frequency 129 (f.sub.SHD+,-)
may be modulated using square-wave frequency modulation, sinusoidal
frequency modulation, or other type of frequency modulation. When
square-wave frequency modulation is used to modulate the first SHD
frequency 123 and the second SHD frequency 129, the modulation
frequencies of the first SHD frequency 123 and the second SHD
frequency 129 may be periodically switched. For example, during a
first half of a switching cycle, the first SHD frequency 123,
f.sub.SHD-,+, may be slightly below the proper heterodyne frequency
and the second SHD frequency 129 f.sub.SHD+,- may be slightly above
the proper heterodyne frequency by the same amount as the first SHD
frequency 123 is below the proper heterodyne frequency. During the
second half of the switching cycle, the first SHD frequency 123,
f.sub.SHD-,+, may be switched to be slightly above the proper
heterodyne frequency and the second SHD frequency 129 f.sub.SHD+,-
may be switched to be slightly below the proper heterodyne
frequency by the same amount that the first SHD frequency 123 is
above the proper heterodyne frequency. By switching the first SHD
frequency 123 and the second SHD frequency 129 as described above,
the first SHD frequency 123 and the second SHD frequency 129 are
rarely equal to one another. Thus, optical backscatter errors that
arise from both the first SHD frequency 123 and the second SHD
frequency 129 simultaneously being at the heterodyne frequency are
suppressed. Additionally, by switching the first SHD frequency 123
and a second SHD frequency 129 as described above, both the
averages of the first SHD frequency 123 and the second SHD
frequency 129 are substantially at or very near the heterodyne
frequency over a period of time. Thus, SHD modulation imperfections
are also suppressed.
In certain embodiments, the RFOG 101 may include a frequency
switching controller 131. As described herein, the frequency
switching controller 131 may control the sideband heterodyne
switching of the first SHD frequency 123 and the second SHD
frequency 129. In certain embodiments, the frequency switching
controller 131 may include digital components, such as a processor
that executes instructions to control the modulation of the first
SHD frequency 123 and the second SHD frequency 129. In alternative
embodiments, the frequency switching controller 131 may include
analog components that periodically switch the modulation
frequencies of the first SHD frequency 123 and the second SHD
frequency 129.
In various embodiments, the frequency switching controller 131 may
provide square-wave modulation of the first SHD frequency 123 and
the second SHD frequency 129. For example, the first SHD frequency
123 and the second SHD frequency 129 may each be equal to 3 MHz
when the FSR is 2 MHz and the SHD frequency is 1.5 times the FSR.
The frequency switching controller 131 may modulate the first SHD
frequency 123 and the second SHD frequency 129 by a frequency
having a magnitude of 100 Hz but opposite signs for the first SHD
frequency 123 and the second SHD frequency 129. The frequency
switching controller 131 may modulate the sideband heterodyne
frequencies such that the first SHD frequency 123 and the second
SHD frequency 129 modulate the respective first optical beam 115
and the second optical beam 117 at different frequencies most of
the time but at the same frequency on average over time. For
instance, during the first half of a modulation frequency cycle,
the frequency switching controller 131 may modulate the first SHD
frequency 123 such that the first SHD frequency 123 is 100 Hz less
than the proper heterodyne frequency. Also, the frequency switching
controller 131 may modulate the second SHD frequency 129 such that
second SHD frequency 129 is 100 Hz more than the proper heterodyne
frequency.
During the transition from the first to the second half of the
modulation frequency cycle, the frequency switching controller 131
may switch the modulation of the first heterodyne frequency 123 and
the second heterodyne frequency 129. Thus, during the second half
of the modulation cycle, the frequency switching controller 131 may
modulate the first SHD frequency 123 such that the first SHD
frequency 123 is 100 Hz above the proper frequency. Also, the
frequency switching controller 131 may modulate the second SHD
frequency 129 such that the second SHD frequency 129 is 100 Hz
below the proper frequency. Accordingly, the first SHD frequency
123 and the second SHD frequency 129 are at different frequencies
most of the time but on average are at the same frequency.
In certain embodiments, in addition to providing the modulation
frequencies for the first SHD frequency 123 and the second sideband
heterodyne frequency 129, the frequency switching controller 131
may also provide a frequency to the CCW resonance tracking
electronics 111 and the CW resonance tracking electronics 147.
Accordingly, the demodulator 109 and the demodulator 139 may each
demodulate received signals at respectively twice the f.sub.SHD-,+
and the f.sub.SHD+,-. In some embodiments, the CCW resonance
tracking electronics 111 and the CW resonance tracking electronics
147 receive the demodulation frequencies from both the frequency
switching controller 131 and respective frequency generators of the
first SHD frequency 123 and the second SHD frequency 129.
Accordingly, the demodulation frequencies used by the demodulators
109 and 139 may be switched in a similar manner to the first SHD
frequency 123 and the second SHD frequency 129. Accordingly, the
RFOG 101 may suppress both modulation imperfection errors and
optical backscatter errors.
FIG. 2 is a schematic diagram illustrating an exemplary RFOG 201.
The RFOG 201 may function similarly to the RFOG 101 described above
in FIG. 1, with the exception that RFOG 201 also implements
resonance switching. For example, the RFOG 201 may include a first
laser source 221, a second laser source 235, a first phase
modulator 213, a second phase modulator 237, a first heterodyne
frequency 223, a second heterodyne frequency 229, couplers 225 and
227, a resonator 249, a first photodetector 219, and a second
photodetector 233 that function similarly to the first laser source
121, the second laser source 135, the first phase modulator 113,
the second phase modulator 137, the first heterodyne frequency 123,
the second heterodyne frequency 129, the couplers 125 and 127, the
resonator 149, the first photodetector 119, and the second
photodetector 133 in FIG. 1. Accordingly, the RFOG 201 provides for
sideband heterodyne modulation as described above in FIG. 1.
As discussed above, the first laser source 221 may be controlled
such that the first optical beam 215 is locked to a resonant mode.
Similarly, the second laser source 235 may be controlled such that
the second optical beam 217 is also locked to a resonant mode. In
some embodiments, the first optical beam 215 may be locked to a
first resonant mode and the second optical beam 217 may be locked
to a second resonant mode, where the first resonant mode and the
second resonant mode are separated from each other by an integer
multiple of the free spectral range. To lock the first optical beam
215 and the second optical beam 217 to their respective resonant
modes, the RFOG 201 may include feedback control that includes a
CCW resonance switching electronics 211 and a CW resonance
switching electronics 247.
In some embodiments, the CCW resonance switching electronics 211
and the CW resonance switching electronics 247 function similarly
to the CCW resonance tracking electronics 111 and the CW resonance
tracking electronics 147 described above in FIG. 1. However, the
CCW resonance switching electronics 211 and the CW resonance
switching electronics 247 are referred to as "resonance switching"
because the embodiments described with reference to RFOG 201
periodically swap the respective resonant modes used for the first
optical beam 215 and the second optical beam 217. That is, after
operating for a fixed period of time with the first optical beam
215 at a first resonant mode and the second optical beam 217 at a
second resonant mode, the resonance switching electronics 211 and
247 will simultaneously switch the resonant mode of the first
optical beam 215 from the first resonant mode to the second
resonant mode and the resonant mode of the second optical beam 217
from the second resonant mode to the first resonant mode. Then,
after operating for the fixed period of time with the first optical
beam 215 at the second resonant mode and the second optical beam
217 at the first resonant mode, the resonance switching electronics
211 and 247 will simultaneously switch the resonant mode of the
first optical beam 215 from the second resonant mode back to the
first resonant mode and the resonant mode of the second optical
beam 217 from the first resonant mode back to the second resonant
mode. Alternating each optical beam between different resonant
modes in this manner further facilitates mitigation of interference
type backscatter error, errors caused by temperature induced
variations in the free spectral range, and line shape asymmetry
grading errors caused by optical backscatter or back-reflections.
Resonance switching is described in greater detail in U.S. Pat. No.
9,772,189, which is titled "SYSTEMS AND METHODS FOR RESONANCE
SWITCHING RESONATOR FIBER OPTIC GYROSCOPES (RFOGS) WITH
FEED-FORWARD PROCESSING", herein incorporated in its entirety by
reference.
In certain embodiments, the CCW resonance switching electronics 211
receive signals from the frequency switching controller 231 and the
first SHD frequency 223 such that the demodulator 209 may
demodulate received signals from the ADC 203 at twice the modulated
first SHD frequency 223 f.sub.SHD-,+. Similarly, the CW resonance
switching electronics 247 may receive signals from the frequency
switching controller 231 and the second SHD frequency 229 such that
the demodulator 239 may demodulate received signals from the ADC
243 at twice the modulated second SHD frequency 229 f.sub.SHD+,-.
The frequency switching controller 231 periodically swap the
modulation of the first SHD frequency 223 and the modulation of the
second SHD frequency 229 as described above with respect to FIG. 1.
Thus, systems within the RFOG 201 periodically switch both the
resonant modes of the first optical beam and the second optical
beam 217 (referred to hereafter as "resonant mode switching") and
the modulation of the first SHD frequency 223 and the second SHD
frequency 229 (referred to hereafter as "sideband heterodyne
switching").
As described herein, the period of the resonant mode switching and
the period of the sideband heterodyne switching may be independent
from one another. For example, the sideband heterodyne switching
may occur at different times than the resonant mode switching.
Alternatively, the period of the resonant mode switching and the
period of the sideband heterodyne switching may be related to one
another, such that either the sideband heterodyne switching may be
limited to occur when resonant mode switching occurs or resonant
mode switching is limited to occur when sideband modulation occurs.
For example, the sideband heterodyne switching period may be equal
to the resonant mode switching period, the sideband heterodyne
switching period may be equal to an integer multiple of resonant
mode switching periods, or the resonant mode switching periods may
be equal to an integer multiple of sideband heterodyne switching
periods.
In certain embodiments, the RFOG 201, or larger system controlling
the RFOG 201, may filter out some or all of the measurements
produced within or by the RFOG 201 when the resonant mode switching
or the sideband heterodyne switching occurs. For example, when the
resonant mode switching or the sideband heterodyne switching
happens, transient signals may be produced that could cause errors
in the measurements produced within the RFOG 201. Accordingly, by
filtering out measurements produced when the resonant mode
switching or the sideband heterodyne switching occurs, the
transient signals may have a limited effect on the measurements
produced by the RFOG 201. Thus, by having the period of the
resonant mode switching and the period of the sideband heterodyne
switching related to one another, there may be less moments when
measurements are filtered.
In some embodiments, to coordinate the periods of the resonant mode
switching and the sideband heterodyne switching, the frequency
switching controller 231 may receive a signal from and provide a
signal to the resonant switching electronics 211 and 247. For
example, the resonance switching electronics 211 and 247 may send
an indicator that resonant modes are switching to the frequency
switching controller 231, where upon the frequency switching
controller 231 may perform the sideband heterodyne switching.
Alternatively, the frequency switching controller 231 may send an
indicator that the sideband modulation switch is occurring to the
resonant switching electronics 211 and 247, whereupon the resonance
switching electronics 211 and 247 may perform the resonant mode
switching. In an alternative embodiment, a processor (not shown)
may direct and coordinate the resonant mode switching and the
sideband heterodyne switching by sending signals to both the
frequency switching controller 231 and the resonance switching
electronics 211 and 247. In a further embodiment, the frequency
switching controller 231 are part of the resonance switching
electronics 211 and 247 or are part of the processor.
FIGS. 3A-3D illustrates several graphs 301a-301d that illustrate
different frequencies of the optical beams as they propagate within
the resonator 249. For example, FIG. 3A illustrates a graph of
different optical beam frequencies that propagate within the
resonator 249 in the CW direction and in the CCW direction. When an
RFOG implements sideband heterodyne modulation, each optical beam
may have a lower sideband frequency, a carrier frequency and an
upper sideband frequency. For example, the optical beam that
propagates in the CW direction may have a portion that propagates
at a lower sideband frequency 305, a carrier frequency 307, and an
upper sideband frequency 309. Also, the optical beam that
propagates in the CCW direction 303 may have a portion that
propagates at a lower sideband frequency 311, a carrier frequency
313, and an upper sideband frequency 315.
Further, a resonator may have one or more resonant modes as
illustrated by the CW resonance graph 301 and the CCW resonance
graph 303. As illustrated, the resonator has multiple resonant
peaks or modes in the CW direction and the CCW direction, where the
distance between adjacent resonant peaks is equal to the free
spectral range for the resonator. As shown, both the CW carrier
frequency 307 and the CCW carrier frequency 313 are located at
frequencies between two resonant peaks. Also, the CW carrier
frequency 307 is separated from the CCW carrier frequency 313 by a
multiple of the free spectral range.
As illustrated, the sideband frequencies may be separated from the
carrier frequencies by one and a half times the free spectral
range. For example, the lower CW sideband frequency 305 may be
located proximate to a resonant mode that is one and a half free
spectral ranges below the CW carrier frequency 307 and the upper CW
sideband frequency 309 may be located proximate to a resonant mode
that is one and a half free spectral ranges above the CW carrier
frequency 307. Similarly, the lower CCW sideband frequency 311 may
be located proximate to a resonant mode that is one and a half free
spectral ranges below the CCW carrier frequency 313 and the upper
CCW sideband frequency 315 may be located proximate to a resonant
mode that is one and a half free spectral ranges above the CCW
carrier frequency 313.
In certain embodiments, the carrier frequencies are modulated such
that the sideband of optical beams are locked away from their
resonant modes by an offset frequency and the opposite propagating
beams are offset in different directions from their respective
resonant modes. For example, the CW sideband frequencies may be
offset from the CW carrier frequency 307 by one and a half free
spectral ranges plus the offset frequency. In contrast, the CCW
sideband frequencies may be offset from the CCW carrier frequency
313 by one and a half free spectral ranges minus the offset
frequency. By separating the sideband frequencies by the offset
frequency, backscatter errors may be suppressed as described above
with respect to FIG. 1.
As described above, an RFOG may implement sideband heterodyne
switching. FIG. 3B illustrates a graph of different optical beam
frequencies that propagate within a resonator 149 or 249 where the
direction of sideband modulation offset for the optical beams
propagating in opposite directions within the resonator has
switched from the direction of sideband modulation described above
in FIG. 3A. For example, when a sideband modulation switch occurs
to the arrangement of sideband frequencies described above in FIG.
3A, the CW sideband frequencies 305 and 309 may be offset from the
CW carrier frequency 307 by one and a half free spectral ranges
minus the offset frequency. Also, the CCW sideband frequencies 311
and 315 may be offset from the CCW carrier frequency 313 by one and
a half free spectral ranges plus the offset frequency. As described
above, sideband modulation switches happen periodically such that
over time each upper and lower sideband frequency for both optical
beams may be at the resonant frequency on average.
Further, an RFOG may implement resonant mode switching as described
above with respect to FIG. 2. FIG. 3C illustrates a graph of
different optical beam frequencies that propagate within a
resonator 249 where the resonant mode has switched for the optical
beams propagating in opposite directions from the resonant modes
described above in FIG. 3A. When an RFOG performs resonant mode
switching, the sideband modulation of the optical beams in the
resonator stays the same, but the carrier frequency for the
opposite propagating optical beams may switch with one another. For
example, the CW carrier frequency 307 and the CCW carrier frequency
313 may switch with one another. Accordingly, the CW carrier
frequency 307 in FIG. 3C is equal to the CCW carrier frequency 313
in FIG. 3A and the CCW carrier frequency 313 in FIG. 3C is equal to
the CW carrier frequency 307 in FIG. 3A.
Further, an RFOG may implement both resonant mode switching and
sideband heterodyne switching as described above with respect to
FIG. 2. FIG. 3D illustrates a graph of different optical beam
frequencies that propagate within a resonator 249 where both the
resonant mode and the sideband modulation have switched for the
optical beams propagating in opposite directions from the resonant
modes and sideband modulation described above in FIG. 3A.
Accordingly, the CW carrier frequency 307 in FIG. 3D is equal to
the CCW carrier frequency 313 in FIG. 3A in the CCW carrier
frequency 313 in FIG. 3D is equal to the CW carrier frequency 307
in FIG. 3A. Also, the CW sideband frequencies 305 and 309 may be
offset from the CW carrier frequency 307 by one and a half free
spectral ranges minus the offset frequency. Also, the CCW sideband
frequencies 311 and 315 may be offset from the CCW carrier
frequency 313 by one and a half free spectral ranges plus the
offset frequency. An RFOG may perform any combination of resonant
mode switches and sideband modulation switches as described above
in FIGS. 3A-3D.
FIG. 4 is a schematic drawing of an additional embodiment of an
RFOG 401 similar to other examples described above that implement
sideband heterodyne switching and, in some implementations,
resonant mode switching. The RFOG 401 may function within a
navigation system, a platform stabilization system, appointing
system, or other system where rotational information may provide a
practical application. For example, in some embodiments, the RFOG
401 may be implemented as part of an inertial sensor unit that
includes one or more RFOGs and one or more linear accelerometers.
The RFOG 401 may measure rotational rate and outputs a signal
indicative of rotation rate. The measured rotation rate from the
RFOG 401 may be used to calculate parameters such as position,
orientation, and angular velocity. The calculated parameters, in
some embodiments, may be used for control signals that are
outputted to one or more optional actuators.
The RFOG 401 may include many similar components to the RFOGs 101
and 201 described above in connection with FIGS. 1 and 2. For
example, the laser sources 421 and 435, the resonance tracking
electronics 411 and 447, the ADCs 403 and 443, the demodulators 409
and 439, the servos 407 and 445, the frequency generators 405 and
441, the phase modulators 413 and 437, the optical beams 415 and
417, the frequency switching controller 431, and the resonator 449
function in similar manners to components having corresponding
numbers (4XX corresponding to 2XX and 1XX) described above in FIGS.
1 and 2.
In some embodiments, optical beams from laser sources 421 and 435
may circulate through the resonator 449. The optical beams may be
coupled into the resonator 449 at the couplers 425 and 427 so that
light can propagate within the resonator 449. In some
implementations, the couplers 425 and 427 may be mirrors, fiber
optic couplers, waveguides, or other suitable component for
coupling light into the resonator 449. In some embodiments, each of
the couplers 425 and 427 may function as both input ports and
output ports for the resonator 449. For example, the coupler 425
may couple light into the resonator 449 in a first direction (e.g.,
clockwise). Similarly, the coupler 427 may couple light into the
resonator 449 in a second and opposite direction (e.g.,
counter-clockwise).
In some implementations, the couplers 425 and 427 may be formed on
a silicon optical bench 489, where the silicon optical bench may
include transmission ports and reflection ports. Also, the silicon
optical bench may include circulators 467 and 493. The circulators
467 and 493 may receive optical beams 415 and 417 from the laser
sources 421 and 435 and circulate the received optical beams 415
and 417 to the couplers 425 and 427 to be subsequently propagated
within the resonator 449. Additionally, the circulators 467 and 493
may receive light from the couplers 425 and 427 that has propagated
within the resonator 449 and provide the light from the couplers
425 and 427 to photodetectors 419 and 433. For example, the
circulator 467 may receive the second optical beam 417 and provide
the second optical beam 417 to the coupler 425, where the coupler
425 couples the second optical beam 417 into the resonator 449.
Additionally, the coupler 425 may couple the first optical beam 415
out of the resonator 449 and provide the first optical beam 415 to
the circulator 467, whereupon the circulator 467 provides the
received first optical beam 415 to the first photodetector 419.
Similarly, the circulator 493 may receive the first optical beam
415 and provide the first optical beam 415 to the coupler 427,
where the coupler 427 couples the first optical beam 415 into the
resonator 449. Also, the coupler 427 may couple the second optical
beam 417 out of the resonator 449 and provide the second optical
beam 417 to the circulator 493, whereupon the circulator 493
provides the received second optical beam 417 to the second
photodetector 433. Additionally, the coupler 425 may couple a
portion of light propagating within the resonator 449 for detection
by a third photodetector 481.
In additional embodiments, the silicon optical bench 489 may
include two transmission ports and a reflection port of the
resonator 449. For example, the signals provided by the first
photodetector 419 and the second photodetector 433 are output
through the two transmission ports. Also, the signal provided by
the third photodetector 481 is output through the reflection port.
In general, light detected at the transmission ports has propagated
through the resonator, whereas at the reflection port there is a
combination of the portion of the resonator incident light that did
not enter the resonator and light that has propagated through the
resonator. For example, the photodetector 419 provides a signal
through a transmission port that is based on light coupled into the
resonator 449 through the coupler 427. The photodetector 433
provides a signal through a transmission port that is based on
light coupled into the resonator 449 through the coupler 425. The
photodetector 481 provides a signal through the reflection port
that is based on light provided by the coupler 425 that includes
both light from the resonator 449 and the circulator 467.
In certain embodiments, light that is coupled into the resonator
449 may be frequency stabilized using the resonator 449 with
feedback control based on light detected at the photodetectors 419
and 433. Further, in some embodiments where a light produced by a
master laser 475 is used to further stabilize the frequency of the
light propagating within the resonator 449, light may further be
stabilized to signals provided by the third photodetector 481.
Feedback control may stabilize optical beams that propagate within
the resonator 449 at both high and low frequencies.
In some embodiments, feedback control may reduce phase noise using
the Pound-Drever-Hall (PDH) technique. A laser source with PDH
feedback control 473 receives a signal from the third photodetector
481 that provides a signal from the reflection port of the silicon
optical bench 489 that is associated with the master laser source
475. The response of the resonator 449 to changes in relative
frequency between optical beams propagating therein and the
resonator 449 may be faster at the reflection port than the
transmission ports. To stabilize the master laser provided by the
master laser source 475 and thus, reduce the phase noise of the
master laser, the master laser source 475 may be locked onto a CW
resonance of the resonator 449 by using the PDH feedback control
473.
In some embodiments, the resonance line shape at the reflection
port may have significantly more asymmetry than the resonance line
shape at the transmission ports. The significantly larger asymmetry
in the reflection port line shape may result in bias errors and
low-frequency drift errors in the relative frequency between the
master laser and the resonator 449. Since changes in line shape
asymmetry may be driven mostly by thermal effects, the frequency
drift errors may typically be at relatively low frequencies. The
bias errors and low-frequency drift errors may be corrected by
using the transmission ports to lock the laser sources 435 and 421
to the resonator 449.
The low relative frequency noise between the master laser and the
resonator 449 may be transferred to the optical beams 415 and 417
by employing phase lock loops 465 and 495 that function to lock the
optical beams 415 and 417 to the master laser produced by the
master laser source 475 using light from the master laser that is
detected at photodetectors 471 and 497. To lock the optical beams
415 and 417 to corresponding resonance frequencies of the resonator
449, transmission mode feedback control may be used to lock the
sidebands of the lasers to the resonant modes of the resonator 449.
Each optical beam that is slaved to the light produced by the
master laser source 475 may have an associated feedback control
that receives a signal from one of the photodetectors 419 and 433
associated with the transmission ports.
In further embodiments, where light produced by the master laser
source 475 is controlled through PDH feedback control 473 and a
master servo 477, the output of the master laser source 475 may be
coupled for detection by the photodetectors 471 and 497 through
couplers 469 and 491. Further, the output of the master laser
source 475 may be coupled into the resonator 449 through the
coupler 463. The master laser source 475 may be configured to
generate light for circulation within the resonator 449. The light
produced by the master laser source 475 may receive PDH feedback
for locking the frequency of the light from the master laser source
475 to the resonance frequency of the resonator 449 as detected by
the photodetector 481. The PDH feedback control 473 may be coupled
to a servo 477 (that also receives a signal associated with the
master laser through the coupler 479 and photodetector 487) that is
coupled to modulate the light produced by the master laser source
475 with the PDH modulation signal and adjust the light produced by
the master laser source 475 towards the resonance of the resonator
449.
As illustrated, the first optical beam 415 and the second optical
beam 417 are locked to the light produced by the master laser
source 475. Locking the first optical beam 415 and the second
optical beam 417 to the light produced by the master laser source
475 may provide the first optical beam 415 and the second optical
beam 417 with a frequency noise reduction of the PDH feedback
control 473. Additionally, locking the first optical beam 415 and
the second optical beam 417 to the light produced by the master
laser source 475 may also provide common modulation to the first
optical beam 415 and the second optical beam 417. For example,
master laser may be modulated with the common frequency at the
phase modulators 483 and 469. The output from the phase modulator
483 may then be coupled to both the optical phase locked loop
electronics 465 and 495 through the couplers 469 and 491 and
photodetectors 461 and 497. The optical phase locked loop
electronics 465 and 495 may then lock the first optical beam 415
and the second optical beam 417 to the commonly modulated master
laser. Further, resonance tracking electronics 411 and 447 may
respectively include common demodulators 451 and 423, that function
to demodulate the signals received from the heterodyne demodulators
409 and 439, which function substantially similarly to the
demodulators 109, 209, 139, and 239 described above in FIGS. 1 and
2.
As illustrated, component of the RFOG 401 may be fabricated on a
multifrequency laser source 453. The multifrequency laser source
453 may include the laser sources 435, 475, and 421 and phase lock
loops and control electronics for the laser sources 435, 475, and
421. In some embodiments the multifrequency laser source 453 may
include a silicon photonics chip (SiP) 457 that includes components
for modulating the optical beams within the RFOG 401. As described
above, the use of sideband heterodyne switching aids in suppressing
optical backscatter errors such that low cost SiP chips (such as
the SiP chip 457) may be used to provide the sideband heterodyne
modulation.
FIG. 5 is a flow chart diagram of a method 500 for performing
sideband heterodyne switching in an RFOG. The method 500 proceeds
at 501, where a plurality of optical beams comprising a first
optical beam and a second optical beam are generated. Additionally,
method 500 proceeds at 503, where heterodyne modulation of the
first optical beam and the second optical beam are performed at a
heterodyne frequency plus a modulation frequency offset to produce
a plurality of sideband optical beams. In some embodiments, the
modulation offset may have a different sign for the heterodyne
modulation of the first optical beam than the modulation of the
second optical beam.
In certain embodiments, the method 500 proceeds at 505, where the
signs of the modulation frequency offset applied to the heterodyne
modulation of the first optical beam and the second alternate beam
is alternated. In some implementations, due to the alternating
modulation frequency offsets, the heterodyne modulation modulates
the first optical beam and the second optical beam substantially on
average at the heterodyne frequency during a period of time.
Further, the method 500 proceeds at 507, where the plurality of
sideband optical beams are circulated within a resonator. Also, the
method 500 proceeds at 509, where the plurality of sideband optical
beams that are transmitted out of the resonator are detected.
Additionally, the method 500 proceeds at 511, where the frequencies
of the generated plurality of optical beams are adjusted based on
the detected plurality of sideband optical beams.
EXAMPLE EMBODIMENTS
Example 1 includes a system comprising: a resonator configured to
allow light to resonate therein; at least one laser source
configured to produce a plurality of optical beams, the plurality
of optical beams comprising a first optical beam and a second
optical beam; a plurality of heterodyne modulators that perform
heterodyne modulation of the first optical beam and the second
optical beam at a heterodyne frequency plus a modulation frequency
offset to produce a plurality of sideband optical beams at a
plurality of sideband frequencies, wherein the modulation frequency
offset has a different sign for the first optical beam and the
second optical beam; a frequency switching controller that performs
sideband heterodyne switching that alternatingly switches the signs
of the modulation frequency offset applied to the first optical
beam and the second optical beam, wherein the heterodyne modulation
of the first optical beam and the second optical beam are on
average substantially at the heterodyne frequency during a period
of time; at least one coupler configured to couple the plurality of
sideband optical beams into the resonator; and a feedback control
coupled to the resonator that detects the plurality of sideband
optical beams transmitted out of the resonator and adjusts
frequencies of the plurality of optical beams based on the detected
plurality of sideband optical beams.
Example 2 includes the system of Example 1, wherein the first
optical beam and the second optical beam are generated to propagate
within the resonator at different resonant modes.
Example 3 includes the system of Example 2, wherein the frequency
switching controller periodically performs resonant mode switching,
wherein the resonant mode switching switches resonant modes of the
first optical beam and the second optical beam.
Example 4 includes the system of Example 3, wherein the frequency
switching controller performs the sideband heterodyne switching and
the resonant mode switching at the same period.
Example 5 includes the system of any of Examples 3-4, wherein the
frequency switching controller performs the sideband heterodyne
switching and the resonant mode switching at different periods that
are harmonically related to one another.
Example 6 includes the system of any of Examples 1-5, wherein the
plurality of heterodyne modulators are located on a silicon
photonics chip.
Example 7 includes the system of any of Examples 1-6, wherein the
feedback control demodulates the detected plurality of sidebands at
a demodulation frequency that is twice of the combination of the
heterodyne frequency and the corresponding modulation frequency
offset.
Example 8 includes the system of any of Examples 1-7, wherein the
modulation frequency offset is applied to the heterodyne frequency
by modulating the heterodyne frequency by an offset frequency.
Example 9 includes the system of any of Examples 1-8, wherein
frequencies of the first optical beam and the second optical beam
are locked to a reference frequency of a master laser.
Example 10 includes a method comprising: generating a plurality of
optical beams, the plurality of optical beams comprising a first
optical beam and a second optical beam; performing heterodyne
modulation of the first optical beam and the second optical beam at
a heterodyne frequency plus a modulation frequency offset to
produce a plurality of sideband optical beams at a plurality of
sideband frequencies, wherein the modulation frequency offset has a
different sign for the heterodyne modulation of the first optical
beam than the modulation of the second optical beam; alternating
the signs of the modulation frequency offset applied to the
heterodyne modulation of the first optical beam and the second
optical beam, wherein the heterodyne modulation modulates the first
optical beam and the second optical beam substantially on average
at the heterodyne frequency during a period of time; circulating
the plurality of sideband optical beams in a resonator; detecting
the plurality of sideband optical beams transmitted out of one or
more ports of the resonator; adjusting the frequencies of the
generated plurality of optical beams based on the detected
plurality of sideband optical beams.
Example 11 includes the method of Example 10, wherein generating
the plurality of optical beams comprises generating the first
optical beam and the second optical beam to propagate within the
resonator at different resonant modes.
Example 12 includes the method of Example 11, further comprising
periodically performing resonant mode switching, wherein the
resonant mode switching switches carrier frequencies of the first
optical beam and the second optical beam.
Example 13 includes the method of Example 12, further comprising
performing the sideband heterodyne switching and the resonant mode
switching at the same period.
Example 14 includes the method of any of Examples 12-13, further
comprising performing the sideband heterodyne switching and the
resonant mode switching at different periods that are harmonically
related to one another.
Example 15 includes the method of any of Examples 10-14, wherein
the plurality of heterodyne modulators are located on a silicon
photonics chip.
Example 16 includes the method of any of Examples 10-15, further
comprising demodulating the detected plurality of sidebands at a
demodulation frequency that is twice of the combination of the
heterodyne frequency and the corresponding modulation frequency
offset.
Example 17 includes the method of any of Examples 10-16, performing
heterodyne modulation of the first optical beam and the second
optical beam comprises modulating the heterodyne frequency by a
respective offset frequency.
Example 18 includes the method of any of Examples 10-17, further
comprising locking the frequencies of the first optical beam and
the second optical beam to a reference frequency of a master
laser.
Example 19 includes a resonator fiber-optic gyroscope comprising: a
resonator configured to allow light to resonate therein, wherein
the resonator has a plurality of resonant modes, each resonant mode
separated by a free spectral range; a first laser source that
produces a first optical beam having a first frequency; a first
heterodyne modulator to modulate the first optical beam at a first
offset heterodyne frequency to produce first sideband signals that
are offset by the first offset from first sideband resonant modes
in the plurality of resonant modes, wherein the first sideband
signals propagate in the resonator in a first direction; a second
laser source that produces a second optical beam having a second
frequency; a second heterodyne modulator to modulate the second
optical beam at a second offset heterodyne frequency to produce
second sideband signals that are offset by the second offset from
second sideband resonant modes in the plurality of resonant modes,
wherein the second sideband signals propagate in the resonator in a
second direction that is opposite to the first direction; a first
feedback control configured to: detect the first sideband signals
received from a first port of the resonator to produce a first
detected signal; demodulate the first detected signal to form a
first demodulated signal; and adjust the first frequency such that
the first sideband signals move closer to being offset by the first
offset from the first sideband resonant modes based on the first
demodulated signal; and a second feedback control configured to:
detect the second sideband signal received from a second port of
the resonator to produce a second detected signal; demodulate the
second detected signal to form a second demodulated signal; and
adjust the second frequency such that the second sideband signals
move closer to being offset by the first offset from the second
sideband resonant modes based on the second demodulated signal; a
frequency switching controller that periodically switches the first
offset to a value of the second offset and the second offset to a
value of the first offset.
Example 20 includes the resonator fiber-optic gyroscope of Example
19, wherein the frequency switching controller periodically swaps
the first sideband resonant modes and the second sideband resonant
modes.
Although specific embodiments have been illustrated and described
herein, it will be appreciated by those of ordinary skill in the
art that any arrangement, which is calculated to achieve the same
purpose, may be substituted for the specific embodiments shown.
Therefore, it is manifestly intended that this invention be limited
only by the claims and the equivalents thereof.
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